Microelectronic applications of Group II-VI semiconductor materials have been widely investigated. In particular, the wide bandgap Group II-VI semiconductor zinc selenide (ZnSe), and its related alloys such as zinc cadmium selenide (ZnCdSe) and zinc sulfur selenide (ZnSSe), are being widely investigated as optoelectronic devices which are effective from the blue to the green region of the visible spectrum. As is well known to those having skill in the art, the wide bandgap of these Group II-VI devices compared to their equivalent nearly lattice matched Group III-V or elemental semiconductor materials, make these wide bandgap Group II-VI semiconductors potential candidates for blue to green optoelectronic devices. Blue to green optoelectronic sources are currently sought for a number of applications, including full color electroluminescent displays, read-write laser sources for high density information storage on magnetic and optical media, sources for undersea optical communications and other applications.
Molecular beam epitaxy and other fabrication techniques have recently been developed so that both n-type and p-type doped layers of zinc selenide and other related II-VI semiconductor materials may be grown. See for example a publication by Ren et al. entitled Substitutional Doping of ZnSe Films, Journal of Crystal Growth, Vol. 111, pp. 772-775, 1991. It has also long been known to make Schottky contacts to n-type zinc selenide using mercury selenide (HgSe). See the publications entitled e, a Highly Electronegative Stable Metallic Contact for Semiconductor Devices by Best et al., Applied Physics Letters, Vol. 29, No. 7, pp. 433-434, 1976; Highly Electronegative Contacts to Compound Semiconductors by Scranton et al., Journal of Vacuum Science and Technology, Vol. 14, No. 4, pp. 930-934, 1977; and Lattice-Matched Heterostructures as Schottky Barriers: HgSe/CdSe by Best et al., Journal of Vacuum Science Technology, Vol. 16, No. 5, pp. 1130-1133, 1979.
As a result of these and other developments, at least four research groups have recently described the fabrication of blue and/or green light emitting diodes and/or laser diodes from Group II-VI semiconductors.
The first group is from North Carolina State University (NCSU) and includes the present inventor. The fabrication of blue and green light emitting diodes based on ZnSe and alloys thereof is described in a publication entitled ZnSe Light-Emitting Diodes by Ren et al., Applied Physics Letters, Vol. 57, No. 18, pp. 1901-1903, October, 1990, and Blue (ZnSe) and Green (ZnSe.sub.0.9 Te.sub.0.1) Light Emitting Diodes by Ren et al., Journal of Crystal Growth, Vol. 111, pp. 829-832, 1991.
A second group of researchers from Brown University and Purdue University have also described zinc selenide based laser diodes and light emitting diodes in publications entitled Blue-Green Injection Laser Diodes in (Zn,Cd)Se/ZnSe Quantum Wells by Jeon et al., Applied Physics Letters Vol. 59, No. 27, pp. 3619-3621, December, 1991; Blue/Green pn Junction Electroluminescence from ZnSe-based Multiple Quantum-Well Structures by Xie et al., Applied Physics Letters Vol. 60, No. 4, pp. 463-465, January, 1992; ZnSe Based Multilayer pn Junctions as Efficient Light Emitting Diodes for Display Applications, Jeon et al., Applied Physics Letters, Vol. 60, No. 7, pp. 892-894, February, 1992; Blue and Green Diode Lasers in ZnSe-based Quantum Wells, Jeon et al., Applied Physics Letters, Vol. 60, No. 17, April, 1992; and Room Temperature Blue Light Emitting P-N Diodes from Zn(S,Se)-Based Multiple Quantum Well Structures, Xie et al., Applied Physics Letters, Vol. 60, No. 16, April, 1992, pp. 1999-2001.
A third group from 3M Company described a zinc selenide based laser diode in an article entitled Blue-Green Laser Diodes by Haase et al., Applied Physics Letters, Vol. 59, No. 11, pp. 1272-1274, September, 1991. A fourth group from the University of Florida and Bellcore described fabrication of LEDs and light emitting diodes using zinc selenide in an article entitled Noncontact Electrical Characterization of Low-Resistivity p-type ZnSe:N Grown by Molecular Beam Epitaxy by Park et al., Applied Physics Letters, Vol. 59, No. 15, pp. 1896-1898, 1991.
The above publications indicate that the art has now successfully fabricated blue and green optoelectrical devices from ZnSe-based materials. As Group II-VI fabrication processes are further refined, optical characteristics such as frequency spectrum width and operational lifetime are expected to improve due to reduced dislocation densities in the materials and other improvements.
A major problem with all of these devices, however, has been the ohmic (nonrectifying) contact to zinc selenide, and in particular to p-type zinc selenide. This is a fundamental problem, which is related to the very deep energy of the valence band of zinc selenide. As a consequence, contacts to p-type zinc selenide and related alloys, using conventional metals such as silver or gold, are not ohmic. In effect, the contacts which have been considered by researchers as being ohmic, actually constitute a reverse biased Schottky diode in series with the device, resulting in a large voltage drop across the supposedly ohmic contact. This large voltage drop results in almost all of the input power to the device being lost as heat. High voltages, of 20-50V or more, have been required in order to induce optical emission, and the resultant heat destroys the devices.
The "ohmic contact problem" for zinc selenide optical emitter devices has been widely reported. See for example the above cited 1990 Ren et al. article from the NCSU group, at page 1901: "It was not possible for us to complete Hall effect studies on the ZnSe:Li samples because of problems associated with non-ohmic contacts." See also the above cited 1992 article by Xie et al. from the Brown-Purdue group, at page 463: "Hall measurements on the p-type layers were unreliable due to the difficulty in forming ohmic contacts to the widegap semiconductor." See also the Haase et al. article by the 3M group at page 1273: "Heating in these samples is a serious problem as the contact between the Au and the p-ZnSe presents a large barrier." Finally, see the Park et al. article from the University of Florida/Bellcore group which notes that ". . . serious problems currently exist with regard to providing low-resistance ohmic contact to p-type ZnSe material . . . "
The above survey indicates that although significant advances have been made in fabricating Group II-VI devices, and in particular zinc selenide based optoelectronic devices, the ohmic contact to these devices, and in particular to p-type zinc selenide, remains a fundamental concern that has heretofore eluded multiple independent groups of researchers.